This invention relates to an apparatus and method for necking smooth die-necked containers. More particularly, the invention uses spinning pilot rollers for necking the containers.
The invention is a modification of the necking apparatus which is described in U.S. Pat. Nos. 4,519,232, 4,774,839, and 5,497,900. As described in those patents, two-piece cans are the most common type of metal containers used in the beer and beverage industry and also are used for aerosol and food packaging. They are usually formed of aluminum or tin-plated steel. The two-piece can consists of a first cylindrical can body portion having an integral bottom end wall and a cylindrical side wall and a second, separately-formed, top end panel portion which, after the can has been filled, is double-seamed onto the can body to close the open upper end of the container.
In most cases, containers used for beer and carbonated beverages have an outside diameter of 211/16 inches (referred to as a 211-container) and are reduced to open end diameters of (a) 26/16 inches (referred to as a 206-neck) typically in a multiple-necking operation for a 206 end; or, (b) 24/16 inches (referred to as a 204 neck) typically in a multiple-necking operation for a 204 end; or, (c) 22/16 inches (referred to as a 202-neck) in a smooth multiple necking operation for a 202 end. Smaller diameter ends can be used, e.g., 200 or smaller, as well as larger diameters, e.g., 209 or 207.5.
As described in the '232, '839, and '900 patents, as the can passes through the apparatus after an initial operation, each of the die necking operations partially overlaps and reforms only a part of a previously-formed portion to produce a necked-in portion on the end of the cylindrical side wall until the necked-in portion extends the desired length. This process produces a smooth, tapered annular wall portion between the cylindrical side wall and the reduced diameter cylindrical neck portion. The tapered annular wall portion which has arcuate portions on either end may be characterized as the necked-in portion or taper between the cylindrical side wall and the reduced diameter neck.
Each container necking operation is performed in a necking module consisting of a turret which is rotatable about a fixed vertical axis. Each turret has a plurality of identical necking substations on the periphery thereof. Each necking substation includes a stationary necking die, a form control member which is reciprocable along an axis parallel to the fixed axis for the turret, and a platform or lifter pad which is movable by cams and cam followers.
An important competitive objective is to reduce the total can weight as much as possible while maintaining its strength and performance in accordance with industry requirements. Accordingly, to minimize the overall container weight, both the side wall and the end panel should be made as thin as possible without compromising the strength and performance of the can. For instance, a top wall thickness of 0.0054 inch in aluminum cans allows a considerable saving on material. However, existing apparatus has difficulty forming a smooth neck of such thickness. Further, it typically takes 16 die necking operations, with an inside can pressurization of 30 psi or more, to reduce the can diameter from a 211 body to a 202 end. The costs of the equipment and the operational costs offset the savings in material.
Spin necking is an alternate method for producing smooth neck configurations. However, it is well known that spin necking, either from the inside or outside of the can, can have problems with stretching and thinning the neck metal and thereby tends to weaken the neck. This stretching of the neck, while tolerable for wall thicknesses considerably larger than 0.0054 inch, is not acceptable for a thickness of 0.0054 inch or lower. Dimensional control of the neck is also an issue with spin necking.
Presently available die necking equipment requires a cylindrical pilot to guide the can edge during the necking operation. However, there is no guidance from the moment the can edge contacts the die to the moment the can edge contacts the pilot. Consequently, wrinkling of the can edge is likely to occur. This can be appreciated, for example, by referring to FIGS. 6-11 of U.S. Pat. No. 4,774,839. Between the time the upper edge of the can contacts the tapered necking portion 204 of the die and the time the can edge contacts the cylindrical pilot 150, the can edge is unsupported and the can wrinkles.
A way of overcoming the above problem is to reduce the gap between the initial can contact with the die and the pilot by increasing the number of necking operations. However, this is very expensive because each necking operation requires a separate necking station. Further, increasing the necking operations does not prevent the forming of minute wrinkles on the edge of the can. Such wrinkles are ironed out by forcing the edge of the can between the cylindrical upper portion of the die and the pilot. Failing to iron out these small wrinkles would allow them to grow in size as the can proceeds from operation to operation.
This ironing operation requires extreme dimensional control of both die and pilot diameters. The gap between the die and the pilot should be uniform around their entire circumferences, preferably about 0.0004 inch more than the thickness of the can wall. Also, forcing the edge of the can between die and pilot requires higher axial forces which tend to crush the body of the can or flatten the bottom of the can. Consequently, the can has to be pressurized to 30 or more psi with compressed air.
To prevent loss of control of the can edge, a pilot shaped over the entire inside profile of the die can be provided. However, once the neck is formed, the can cannot be removed from the pilot. Methods have been developed to expand a pilot during the necking operation to keep the edge of the can in contact with the die and to return the pilot to its original size for can removal. So far, such methods have not been successful for commercial production.